A continuing need in the pressure sensitive adhesive (PSA) art is achievement of a better control over various mechanical and process properties so that adhesives can be "tailor-made" for specific, highly demanding end-use applications such as packaging, medical, and masking tapes. These applications require a proper balance of PSA properties, which varies with each of the different end-uses.
Pressure sensitive adhesive compositions suitable, for example, for use in adhesive tape must have a requisite fourfold balance of adhesion, cohesion, stretchiness and elasticity as disclosed by U.S. Pat. No. Re 24,906, The desire to maintain this balance of properties makes it extremely difficult to improve internal strength i.e., cohesiveness without sacrificing other desirable properties and destroying the overall pressure-sensitive nature of the adhesive system.
Among the earliest polymers to provide a reasonable balance of the properties required for satisfactory PSA performance were the natural and synthetic rubbers. However, these PSAs had poor cohesive strength, especially at elevated temperatures and poor aging properties, e.g. they oxidized easily.
Phase-separating/segregating (PS) block copolymers, like the styrene-butadiene-styrene KRATON.TM. rubbers, can be used to improve the cohesive strength and hot-melt processability of PSAs. However this class of PSAs still has poor aging properties. The PS block copolymers contain end-blocks which are capable of forming phase-separated/segregated "glassy" domains that act as thermally reversible crosslinks providing cohesive strength when such block copolymers are used in pressure-sensitive adhesives. At high temperatures the glassy domains effectively "dissolve" in the rubbery phase. When the polymer is cooled, the domains reform allowing recovery of the original physical and chemical properties. The reinforcing nature of phase-separation in polymers is described in more detail by D. Satas, "Handbook of Pressure-Sensitive Adhesive Technology", Van Norstrand Reinhold, NY, 1982, pp.220-223 and L. H. Sperling, "Introduction to Physical Polymer Science", John Wiley & Sons, NY, 1986, pp. 111-116 & pp. 279-280 which is hereby incorporated for reference.
With the advent of Ziegler-Natta (ZN) catalysts, the polymerization of .alpha.-olefins to polymers, some of which are naturally tacky and have PSA properties, became possible. However, unmodified .alpha.-olefin polymer PSAs generally have poor internal strength.
U.S. Pat. No. 3,542,717 describes laminating adhesives made from tackified mixtures of ZN copolymers derived from .alpha.-olefin monomers of from 4 to 20 carbon atoms. When tackified, the copolymer mixture has PSA properties, while certain other compositions functioned as hot-melt adhesives, see column 4, lines 22-34. While the cohesive strength of the adhesive was acceptable for its intended laminating applications, nothing is mentioned about shear strength at elevated temperature.
U.S. Pat. No. 3,954,697 discloses that PSAs provided by copolymers of polypropylene and C.sub.6 to C.sub.10 .alpha.-olefins can have good cohesive strength when hot-melt coated at a melt temperature of at least 350.degree. F. (177.degree. C.), a temperature at which the copolymers exhibit no detectable crystallinity when examined using X-ray or DSC techniques.
U.S. Pat. No. 4,178,272 discloses that a hot-melt adhesive that provides strong T-peel and lap shear bonds can be made using .alpha.-olefin polymers. The hot-melt adhesive is a blend of poly(propylene-co-higher 1-olefin), tackifying resin, and crystalline polypropylene. The blend is not said to be naturally tacky or a PSA. In Example 1, adhesive bonds are formed at 200.degree. C.
The development of techniques of graft copolymerization, i.e. the attachment of high molecular weight pendant side chains to the polymer backbone, permitted modification of polymer properties. Most of this prior art does not deal with PSA systems.
U.S. Pat. No. 4,007,311 shows that grafting methyl methacrylate to a styrene-isoprene-styrene block copolymer enhances adhesion without regard for elasticity or cohesiveness.
U.S. Pat. Nos. 4,554,324, 4,551,388 and 4,656,213 describe copolymers having macromonomers grafted to an acrylate polymer backbone by free-radical polymerization to improve the shear adhesion of pressure-sensitive adhesives and sheet materials coated therewith.
U.S. Pat. No. 3,862,267 teaches how to make and use a number of vinyl terminated polystyrene macromonomers in copolymerization processes with other ethylenically unsaturated monomers. Initiation of these copolymerizations is described as: addition, anionic, cationic, condensation, and coordination.